Draw a rectangle covering the entire page, then type "/2" after its width and height in the Control panel.

Draw a rectangle frame (the one that shows a big 'X' through it) covering the entire page.

Draw a line from the top left to the bottom right corners, set the proxy to its center, click on the Horizontal (or vertical) flip button with Alt down.

Set the ruler origin to top left of the page, then drag a horizontal guide on the right edge of the page. The guide is still selected when you release the mouse button, and its x-coordinate is in the control panel. Type "/2" after this coordinate and press Return. Repeat for vertical.

Draw a text frame covering the entire page. Set its paragraph alignment to Centered, and the text frame options to Vertically centered. Insert a bullet -- its top center will be the page center.

(CS4) Draw a rectangle, starting from any corner, down to the approximate center of the page -- with Smart Guides enabled, the dragged corner will display big fat smart guides, and the corner will snap into place.

what's his name coud script it, but, the long way is to use those math features that are so much fun in the control bar.

Hit alt+ctrl+p to bring up the document setup (I'm assuming it is not easily memorized here), copy the width dimension to the clipboard.

Drag a guide (or a frame with center proxy selected) and paste the clipboard contents into the x field along with the / (division symbol) 2. Effectively you are telling ID to use value n/2. (n divided by 2).

Tough to explain, here's a screen shot. I set up a A4 document which I never would, and set my preferences to points, which I almost never do...

P.S. Why do you want to know the exact centre? Or to put it another way, what are you going to do with that information once you have it? Knowing that will let us give you a better answer as to the fastest way to find it.

It does. Internally, InDesign does not 'do' millimeters, all measurements are stored in points. Displaying a pt measurement into mm needs a calculation: mm*72.0/25.4, and then it gets rounded, because the floating point decimals inside ID only have about 16 significant digits.

The interface is friendly -- it displays, what? 2 or 3 decimals after the period, although on occasion it may suddenly decide to show the full number (I'm looking now at an image that has been scaled to "15.363379894914805%").

Method #9: create a blank image in Photoshop. Draw a single black pixel in the center. Import into InDesign, scale up to page size. The black pixel will be in the exact center of your page (rounded off to the nearest scaled pixel size).

The interesting factoid to glean from this is Illustrator seems to have only 1/100,000,000th of InDesigns accuracy. Perhaps it's time for the Illustrator programmers to update it to 64-bit floating point numbers.